Recently, active research into an organic substance such as conjugated conducting polymer have been made, since an organic electroluminescent device using poly (p-phenylene vinylene) (PPV), which is a conjugated polymer, was developed. Also, research into applying such an organic substance to a thin film transistor, a sensor, a laser, a photoelectric device, etc., and particularly to an organic electroluminescent device, have been progressing continuously.
Generally, an organic electroluminescent device comprises a multi-layer structure in which thin films composed of different organic substances are disposed between two counter-electrodes so as to increase the efficiency and the stability of the device. As shown in FIG. 1, the most typical multi-layer structure of an organic electroluminescent device comprises a hole injection layer 3 to which holes are injected from an anode 2, a hole transporting layer 4 for transporting holes, an emitting layer 5 in which combinations of holes and electrons are accomplished, and a cathode 7. Such organic electroluminescent device may utilize the said layers composed of mixed materials or further comprise an additional layer for the purpose of improving the efficiency and the life of the device. Additionally, in order to simplify the manufacture of the device, a multi-functional material may be used to reduce the number of the layers contained in the device.
Meanwhile, one electrode on a substrate uses a transparent material having a low absorbance to visible light so as to emanate the light emitted from an organic electroluminescent device to outside, wherein Indium Tin Oxide (ITO) is generally used as a transparent electrode material and as an anode for injecting holes.
An organic electroluminescent device works according to the following mechanism. Holes and electrons generated respectively from an anode having a high work function and a cathode having a low work function are injected into an emitting layer through a hole injection layer/a hole transporting layer and an electron injection layer, thereby producing excitons in the emitting layer. Finally, when the excitons decay, lights corresponding to the energy concerned are emitted.
Researches into an organic electroluminescent device have been made mainly in regard to the efficiency, the life, the driving voltage and the color of light of the device. Particularly, charge injection on the interface between an organic electroluminescence substance and an electrode mostly affects the efficiency and the lifetime. Therefore, intensive researches for improving the interfacial properties have been made.
More particularly, ITO surface treatment methods for improving the interfacial properties between an ITO surface and a hole injection layer are known. Conventionally, ITO surface treatment methods include cleaning by ultrasonification and/or UV ozone, plasma treatments, or the like. Among them, oxygen plasma treatments improve the efficiency and the lifetime of an organic electroluminescent device. See C. C. Wu et al., Applied Physics Letter, 70, 1348, 1997. It is reported that oxygen plasma-treatment on ITO surface makes the work function and sheet resistance of ITO increase and makes ITO surface more uniform. See S. Fujita et al., Japanese Journal of Applied Physics, 36, 350, 1997 and J. S. Kim et al., Journal of Applied Physics 84, 6859, 1995. Also, oxygen plasma treatments on ITO improve the hole injection by increasing a work function of ITO. Further, treatments with oxygen plasma can remove polluting materials present on the ITO surface, thereby improving the performance of an organic electroluminescent device.
Additionally, the interface between an inorganic oxide, i.e., ITO electrode and an organic hole injection layer has a relatively unstable structure compared to a general interface between organic substances. In order to solve this problem, as shown in FIG. 2, a hole tunnel layer (a hole tunneling (buffer) layer) may be inserted between ITO and a hole injection layer so as to improve the adhesion of the organic layer and the hole injection.